Huey, Dewey and Louie Monsters are Huey, Dewey and Louie who are transformed into monsters. They get transformed when they fall into the slime along with Honker Muddlefoot. They eventually revert to normal form when they get hit by the antidote gas.
Huey, Dewey and Louie transformed into three classic Disney monsters: Monstro for Dewey, Maleficent in her dragon form for Louie and Chernabog for Huey. They still kept their clothes when they fell into the slime (except Huey, who lost his shirt but still has his hat when transformed).